1. Field of the Invention
The invention relates to a method and to an apparatus for determining the specific surface s.sub.NMR of a porous or a powdery substance by measuring the spin-lattice relaxation rate of exchanging protons of an impregnating agent by means of nuclear magnetic resonance.
The specific surface s.sub.NMR determined by the present invention method corresponds to the surface of outwards open pores per mass unit of the examined substance, which surface has been moistened by the impregnating agent. Consequently, the surface s.sub.NMR is equal to the surface of a monomolecular impregnating agent layer still existing in the specimen of the examined substance after removing the impregnating agent for a long time. This removal of the impregnating agent is accomplished by evaporation at a pressure below the saturated vapor pressure of the impregnating agent at a given temperature. This is of great importance since it makes the method of the invention for an order of magnitude faster than the known method. Therefore this method appears appropriate for the investigation of diverse surface processes, such as the hydratization of cement. A measuring vessel with the specimen is connected to a vacuum pump system and is placed into a thermostatically controlled probe of a NMR spectrometer.
2. Description of the Prior Art
There has been known a method for determining the specific surface of a porous or a powdery substance by measuring the spin-lattice relaxation rate of impregnating agent protons, as described in the article "The determination of Surface Development in Cement Pastes by Nuclear Magnetic Resonance", J. Am. Cer. Soc., 65 (1982), 25-31.
A specimen is prepared from the examined substance (mass m), which substance is impregnated by an impregnating agent (mass m.sub.i) to an impregnation level m.sub.i /m. The impregnating agent should neither chemically react with the examined substance not get occluded by it. As impregnating agents there are used for example, H.sub.2 O, C.sub.2 H.sub.5 OH, C.sub.6 H.sub.12, C.sub.3 H.sub.7 OH and other liquids. The specimen of a porous substance is prepared by impregnating the substance in a vacuum (m.sub.i /m.congruent.0.1), the specimen of a powdery substance, however, is prepared by kneading a paste after it has been impregnated (m.sub.i /m.congruent.0.1.div.0.5). The specimen is placed into a test cylinder, which is preferably punctured in order that the exhalation of the impregnating agent may be intensified. The effective relaxation time T.sub.1,ef of impregnating agent protons is measured with a NMR spectrometer with coherent pulses at several impregnation levels m.sub.i /m lying within a linerly shaped part of a curve T.sub.1,ef.sup.-1 vs. (m.sub.1 /m).sup.-1 and at very low impregnation levels m.sub. i /m after a long time exhalation when the effective relaxation time T.sub.1,ef is approaching the relaxation time T.sub.1b of the protons of the impregnating agent molecules, which remained in a monomolecular layer adsorbed to the pore surface.
The specific surface s.sub.NMR determined by nuclear magnetic resonance is expressed by the slope of the linearly shaped part of the curve T.sub.1,ef.sup.-1 vs. (m.sub.i /m).sup.-1 : ##EQU1##
S.sub.1 means the surface covered by one molecule of the impregnating agent with the molar weight M.sub.i and N.sub.L is the Loschmidt number.
The impregnating agent is removed from the specimen by exhalation, which makes this process rather lengthy. The exhaled vapor is condensed on the test cylinder walls, which makes impossible a precise measurement of the mass m.sub.i of the impregnation agent still remaining within the specimen and being non-uniformly distributed therein. It is a disadvantage of the described method that it is not suitable when the proton relaxation rate of the impregnating agent "in situ" is not negligible with respect to the proton relaxation rate of the impregnating agent within the adsorbed layer, since in this case the expected accuracy cannot be achieved.